The present invention relates to methods and devices for measuring airborne particles. In particular, the present invention relates to a method and apparatus for determining the concentrations of airborne particles in an aerosol as a function of aerodynamic particle size.
The control of industrial and other environments increasingly requires ongoing knowledge of the quantities of airborne particles in the atmosphere. Dust and other non-gaseous particles are found suspended in air or other gaseous media. Such particles suspensions are referred to herein as aerosols. The effects of airborne particles as well as the control of airborne particles are dependent on the ways that the different particles move.
The motion in air of airborne particles and the deposition of such particles onto surfaces depends primarily on physical particle size. However, density, shape, surface properties and other characteristics of the particles also influence their airborne behavior. Usually these characteristics as well as all characteristics that affect particle motion and deposition are combined with the physical particle size into a property referred to as xe2x80x9caerodynamic particle diameterxe2x80x9d or xe2x80x9caerodynamic particle sizexe2x80x9d. A common method for determining the size distribution of particles in a volume of air, or the xe2x80x9caerosol size distributionxe2x80x9d of the particles, is to collect the airborne or xe2x80x9caerosolxe2x80x9d particles on a filter and to subsequently size them under an optical or electron microscope. However, this method only yields the physical size, not the aerodynamic particle size. To determine how these particles behave in air, in addition to determining the physical particle size by this method, it would be necessary to determine the other characteristics of the particles that will affect their airborne behavior. This technique is time consuming. Therefore, methods of dynamically measuring the sizes of particles dynamically, while suspended in air, have been preferred.
A common method for dynamically measuring the concentration and size distribution of particles in the airborne state uses techniques such as optical single particle size spectrometry. In this method, one particle at a time is passed through an illuminated view volume, and the magnitude of light scattered by each particle is recorded. An optical single particle counter when used with this method is usually calibrated with spherical, monodisperse test particles of known particle density and optical characteristics, such as polystyrene latex (PSL) spheres. However, most airborne particles have their own light scattering and absorption characteristics, so that the xe2x80x9coptical particle diameterxe2x80x9d measured by single particle size spectrometry generally does not correspond to the xe2x80x9caerodynamic particle diameterxe2x80x9d.
When using the optical single particle counting method, the device embodying this method can be dynamically calibrated to measure aerodynamic particle size by placing an impaction stage at its inlet. An impaction stage eliminates, from the aerosol, particles having an aerodynamic size above an aerodynamic threshold or xe2x80x9caerodynamic cutxe2x80x9d. The aerodynamic cut of an impaction stage can be determined by theory or experiment. Through this type of calibration, each optical particle size of the optical single particle counter can be related to its equivalent aerodynamic particle size. To operate an optical single particle counter in the field, the impaction stage is removed from the inlet. When an optical single particle counter is calibrated over a wide particle size range, several impaction stages with different aerodynamic cut sizes are successively attached to the optical single particle counter""s inlet. This calibration is done for each aerosol of a different anticipated airborne particle composition. Such a calibration process is very time consuming.
A simpler dynamic optical particle sensing method is xe2x80x9caerosol photometryxe2x80x9d. In an aerosol photometer, the optical view volume is larger than in an optical single particle counter, thus accommodating a cloud of particles. However, the output from such a device depends not only on the optical characteristics of the particles, but also on the size distribution of all of the particles in the view volume. The light scattered by each particle depends on its refractive index and size.
Another dynamic, but more complex and expensive method for determining the aerodynamic particle size distribution in situ is by accelerating the aerosol particles in a nozzle and then measuring the xe2x80x9ctime of flightxe2x80x9d of each particle between two points. In a nozzle or other acceleration field, aerodynamic drag accelerates large particles to a lesser extent than small particles. Thus, in an acceleration field, the time of flight between two points is longer for large particles than for small particles. The difference is caused by the difference in aerodynamic drag. Thus, this method determines the particle size distribution through a number of different techniques and geometric arrangements, as illustrated, for example, in U.S. Pat. Nos. 4,633,714; 5,296,910; 5,561,515; 5,679,907; and 5,701,012.
Based on the above, there remains a need for a more efficient and more easily used method and apparatus for measuring the concentrations of airborne particles, and particularly for determining the aerodynamic particle size distribution in an aerosol.
An objective of the present invention is to determine the concentrations of airborne particles in an aerosol as a function of aerodynamic particle size. A further objective of the present invention is to provide a method and apparatus by which airborne particle concentration in an aerosol can be simply, economically and rapidly determined.
A particular objective of the present invention is to provide a method and apparatus for dynamically measuring concentrations of airborne particles in an aerosol as a function of their aerodynamic particle sizes or diameters.
According to the principles of the present invention, an aerosol, or air that contains airborne particles, is passed through an aerodynamic cut device which performs an aerodynamic cut of the particles entering an aerosol particle sensor. The airborne particles in the aerosol can be any inert or viable airborne particles, examples of which include dust, fume, smoke, fog, mist, bacteria, pollen, fungal spores, fragments of biological or non-biological material and other non-gaseous particles, or combinations thereof. Preferably, the aerodynamic cut is achieved by a rotating element in the aerodynamic cut device that centrifugally removes from the aerosol particles above or below a specific aerodynamic cut size.
In accordance with the preferred embodiment of the invention, the aerodynamic cut is achieved by a rotating element in the aerodynamic cut device which centrifugally removes particles above a predetermined or selected aerodynamic particle size from the aerosol, preferably to a collecting surface within the aerodynamic cut device. The rotating element may include one or more disc or propeller blades mounted on a rotating shaft, or an impeller with radial, forward-curved or backward-curved blades or other structure that accelerates particles outwardly from the axis of the rotating element. The collecting surface may be an inner wall of the housing containing the rotating element or a rotating enclosure or may be an enclosure that contains the rotating blade or other such structure and which rotates with such structure about its axis of rotation.
In certain preferred embodiments of the present invention, the aerodynamic cut is continuously or intermittently varied by variably controlling the configuration or motion of the rotating element of the cut device so as to change the distribution of particles in the aerosol entering an inlet of an aerosol particle sensor. Preferably, the variation of the rotating element is achieved by operating the element at a sequence of rotational speeds, preferably a sequence in which the rotational speeds are varied from zero through a plurality of discrete increasingly higher rotational speeds. Variation may also include sequentially or continuously adjusting the positions of the blades or other components of the rotating element so as to differently affect the centrifugal motion of the particles in the aerodynamic cut device.
As a result of the variations, a series of aerosols containing particles of sizes larger than the aerodynamic cuts determined by the set rotational speeds or other cut settings of the cut device are measured. The series of measurements are taken with an aerosol sensor of the particles remaining in the aerosol after each cut. Each of the measurements may, in this way, include a single analog measurement of the total particle content or concentration remaining in the aerosol after the cut. Preferably, a controller is provided having a memory that stores the series of measurements and having a processor that combines the data from the different measurements to calculate the concentrations of airborne particles as a function of their aerodynamic particle sizes.
Preferably also, dynamic calibration is carried out before measurements of unknown concentrations of particles are taken. The calibration may include the operation of the rotating element at each of its different settings, such as by increasing its rotary speed through a sequence of predetermined rotational speeds, and collecting or otherwise measuring the total of the particles or the numeric distributions of the remaining particles of each of the aerodynamic size distributions as the parameters or settings of the rotating element are varied. Determination of the aerodynamic particle size distribution may be made by use of a relatively inexpensive sensing device, such as an optical single particle counter, an aerosol photometer, or any other aerosol sensor that responds to a specific property of the aerosol particles such as mass, fluorescence, magnetism, radioactivity, etc.
In one preferred method of the present invention, airborne particles are drawn through an aerodynamic cut device which continuously or intermittently changes the aerodynamic cut of the particles entering an aerosol sensor. Aerosol sensing may include optical sensing performed, for example, by optical single particle size spectrometry or by aerosol photometry. The aerodynamic cut may be achieved by rotating the rotating element to centrifugally remove particles larger than a specific size to the inner wall of the housing containing the rotating element. Continuously changing the aerodynamic cut has the advantage of taking less time, since the voltage signal of the aerosol sensor can be related to the operational characteristics of the aerodynamic cut device through consideration of the time it takes for the aerosol particles to move from the aerodynamic cut device to the sensing element of the aerosol sensor.
In the preferred embodiment of the invention, the rotating element is configured to allow the aerosol to remain in the cut device for sufficient time to allow particles that are to be extracted by the cut device to be efficiently removed from the aerosol. This is achieved by restricting the outlet from the apparatus so as to prevent a high flow rate of aerosol through the apparatus due to the rotation of the rotating element, particularly when the rotating element is rotating at higher rotational speeds. Further, a pump is preferably provided at the outlet of the apparatus to facilitate flow of aerosol through the apparatus at low rotational speeds of the rotating element and when the element is not rotating. Preferably, outlet flow restriction and separate pumping are coordinated to provide a constant rate of aerosol flow through the apparatus.
In the preferred method, the measurements are first taken with the rotating element of the aerodynamic cut device not rotating. When the rotating element is stationary and an optical single particle counter is used as a sensor, the sensor records the aerosol particle counts in specific xe2x80x9coptical particle diameterxe2x80x9d ranges. The amount of light scattered or absorbed by a particle depends on the optical characteristics of the particle. Such xe2x80x9coptical particle diameterxe2x80x9d generally does not correspond to the xe2x80x9caerodynamic particle diameterxe2x80x9d.
Calibration to aerodynamic particle size of an apparatus that employs a single particle counter as a sensor is performed while a constant aerosol particle size distribution is maintained, with the aerodynamic cut device continuously or intermittently increasing its rotational speed as a series of measurements are taken. Each rotational speed corresponds to a specific aerodynamic cut size. In the preferred embodiments of the invention, the aerodynamic particle size of the changing aerodynamic cut decreases as the rotational speed increases. In this way, the optical particle size can be related to a corresponding aerodynamic particle size for the aerosol being measured. Once an optical single particle counter is calibrated relative to aerodynamic particle size for a specific aerosol environment, the aerodynamic cut device can be removed and the optical single particle counter can be operated alone as long as the material characteristics of the aerosol particles and the determined size distribution of particles in the aerosol do not change substantially.
In a preferred embodiment of the invention in which an aerosol photometer is used as the aerosol sensor in the apparatus, the aerodynamic cut device becomes an integral part of the photometer, effectively turning the aerosol photometer into an aerosol size spectrometer. With such an apparatus, at the beginning of a measurement cycle, a measurement is made with the rotating element in the aerodynamic cut device not rotating. Thus, the aerosol photometer output from a measurement of the aerosol entering the aerosol photometer at the beginning of the measurement cycle corresponds to the entire aerosol cloud passing into the apparatus. As the rotating element in the aerodynamic cut device increases its rotational speed, the aerosol photometer output is reduced by the amount of aerosol particles removed in the aerodynamic cut device. Thus, the difference in signal output obtained when increasing the speed of rotation of the rotating element corresponds to the aerosol particles removed in the aerodynamic particle size range defined by two rotating speeds, and is calculated by subtracting the two stored measurements with a controller. For calibration, the aerosol particles that have passed through the aerosol photometer can be collected on a filter, and the particle count, surface area, volume or mass of particles deposited on the filter can be related to the photometer signals.
With this invention, higher aerosol concentrations can be measured than with the more complex and expensive xe2x80x9ctime of flightxe2x80x9d method, because in the xe2x80x9ctime of flightxe2x80x9d method the processing time for each particle is much longer. When aerosol photometry or dynamic aerosol mass sensing is used for the aerosol sensing, the aerosol flow rate can be much higher than in the xe2x80x9ctime of flightxe2x80x9d method. Thus, a much larger volume of air can be analyzed as to its aerodynamic particle concentration in specific aerodynamic size ranges.
Thus, advantages of the present invention include provision of an inexpensive apparatus and method for calibrating an optical single particle counter with respect to the aerodynamic particle sizes of the aerosol particles being sampled. Embodiments of the invention are effective when using an optical single particle counter. The method can be used as an integral part of an aerosol photometer or aerosol mass sensor so that the output of the aerosol photometer or aerosol mass sensor can be related to specific aerodynamic particle sizes, thus turning the aerosol photometer or aerosol mass sensor into an aerodynamic particle size spectrometer.
Another advantage of the present invention is the ability to measure higher concentrations of aerosol particles than is possible with the more complex and expensive xe2x80x9ctime of flightxe2x80x9d method. A further advantage of the invention is the ability to measure a larger volume of aerosol particles per unit of time than is possible with the more complex and expensive xe2x80x9ctime of flightxe2x80x9d method.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the of the preferred embodiments of the invention.